Method of treating alloys



Patented July 21, 1936 METHOD OF TREATING ALLOYS Norman B. Pilling, Elizabeth, N. J., and Paul D.

Merica, New York, N. Y., assignors to The International Nickel Company, Inc., New York, N. Y., a corporation of Delaware No Drawing.

1 Claim.

This invention relates to a method of treating alloys of the solid solution type containing nickel, and more particularly to so-called austenitic steels containing nickel and the development of high strength properties in such alloys.

Hitherto it has been proposed to utilize the metal titanium as a deoxidizing agent for alloy steels and the like in which the residual content of titanium contemplated was very small, usually less than .1 percent. It has been further proposed to use titanium as a toughening agent or grain refiner in which cases the alloy may have some 1 percent of titanium retained, although several disclosures specify ranges of titanium for such purposes up to 10 percent. It is an object of the present invention to provide improved hardenable nickel alloys by combining with a suitable alloy, referred to as the base alloy, quantities of titanium and titanium-like elements.

It is a further. object of this invention to confer hardening properties upon particular base alloy compositionschosento provide other desirable properties, whereby not only the hardness but the elastic strength and breaking strength of the base alloy is increased without materially changing its other characteristic properties.

It is a still further object of this invention to alloy a suitable hardening agent with anickelbearlng base material and subject the resulting alloy to a particular heat treatment to develop and control increased strength properties. These and other desirable advantages of the present invention will be set forth and described in the accompanying specification, certain preferred compositions being given by way of example only,

for, since the underlying principles may be applied to other specific compositions, it is not intended to be limited to those herein shown except as such limitations are clearly imposed by the appended claim.

The present invention comprehends a wide variety of base alloy compositions and three preferred hardening agents, as will be described more in detail hereinafter. The preferred base alloy which is particularly amenable to the proposed treatment may be defined as nickel-bearing solid what in hardness with heat treatment.

Application August 31, 1931, Serial No. 560,511

ceptions to this definition have yet been encountered, although the degree of hardening displayed by difierent combinations of base alloy and hardening agent, of course, vary somewhat in degree. In one such series, viz., iron-nickelchromium-titanium, the hardening characteristics were displayed in alloys having ranges of nickel content varying from substantially 6 to 95 percent.

The preferred hardening agents comprehended within the spirit and scope of this invention are titanium, aluminum, and zirconium, and it is apparent that the hardening characteristics herein disclosed may'be properties or functions of the boron and the titanium groups of the periodic classification of the elements according to Mendeleff. Of these hardening agents titanium has been found to be the more useful from the standpoint of developing physical properties of engineering value combined with practical working qualities.

For purposes of illustration, in order to more clearly set forth the novel features of the present invention, the characteristics of iron-nickel base alloys alloyed with titanium as a hardening agent will be discussed. Nickel-iron alloys which include from about 25 percent to substantially 100 percent nickel in their composition are soft and relatively unaiiected in hardness by heat treatment. Titanium is' soluble in these alloys and, if

completely dissolved therein, the resulting ternary through this range, a substantial rise in hardness occurs. A still further increase in titanium content causes the alloys to become increasingly hard, even when subjected to rapid cooling from high temperatures, yet these alloys change some- These characteristics in a series of iron-nickel-alloys containing percent nickel and varying amounts of titanium are shown in the following table:

Number 1 2 a 4 5 6 1 Percent tltanium Brine {l000 0., air cool.

700 C., a lll'S.W851 AHIlcheHIIIIIIII solutions having the face-centered cubic lattice type of crystalline structure. The claim for this broad definition is predicated on experimental The desirable range of titanium to be added to this particular base is from substantially 1 percent, at which point hardening begins, to about 4 percent, at which point themalleabllity of the alloys becomes impaired. The hardened alloys in common with iron-nickel alloys generally are characterized by their toughness, resistance to attack by non-oxidizing acids, ferro-magnetism As has been intimated herelnbefore, desirable and high electrical resistivity. With an increase results may be obtained by substituting alumiin the nickel content of the base alloy, the denum and/or zirconium for the titanium. In the sirable range of titanium, as just defined, refollowing,table a few typical alloys are givenby 5 mains substantially the same up to 75 percent way of example. 5

nickel/content, but the capacity for hardening displayed by the alloys under consideration, a m r; 21 22 23 24 steadily diminishes with increase in nickel content up to 99% with a range of about 150 to 225 Brinell hardness units. Within the range of 75 to 96 percent nickel content, the minimum titanium content necessary to develop hardening, increases from about 1 percent to somewhat more than 4 percent, the amount being roughly Soft 1000 0. l5 proportional to the excess of nickel over '75 per-. Brinellhardnessnumber. fi 'gg ig ai gl 154 155 172 157 \5 cent. Within this range the hardness differentemp.).. 213 256 232 133 tial developed by heat treatment is from about 75 Substantially 100 Brinell units. In the case of aluminum, the content of this Titanium When d d to y other nickel element necessary to develop suitable hardening alloys ofthe face centered cubic lattice type preresponse varies from about 2.5 to substantially 6 0 viously noted, permits the formation of alloys percent, the latter percentage marking the aphaving hardening characteristics similar to the proximate upper limit of forgeability. A preiron-nickel-titanium alloys described. Among ferred range is from 5.0 to 5.5 percent. these other base alloys may be mentioned: Ironwhen titanium is used as an alloying element, 25 nickel-copper; iron-nickel-chromium; ir o nthe use of commercial ferro-titanium may intro- 25 nickel-manganese; nickel-copper; nickel-chr0- duce appreciable quantities of aluminum and silimium, and nickel metal. The following table con into the metal, both of which elements will shows several malleable alloys exemplifying this appear in the resulting alloy. This content of fact, the hardness numbers being expressed in aluminum is not harmful and it has now been Brinell units: found in fact that the use of even higher con- 9 3.0 5'6 Holt (1000 0. water quench) 130 127 157 159 154 153 152 126 194 165 300 164 178 40 Hard (tempered 600700 C.).. 237 271 302 315 315 304 284 266 284 317 321 200 235 J 40 The ranges of the several elements in additents of aluminum in combination with titanium tion to titanium may be extended as follows: as hardening agents offers certain advantages, copper .5-40%, chromium 3-30%, nickel 2-9 9 notably in accelerating the rate at which the and iron 2-90%, the titanium being replaceable, hardening reaction occurs. As an example of I under the conditions discussed more in detail this discovery, the nickel-iron titanium alloy in- I hereinafter, by from .5-10% of titanium-like cluding 34.8 percent nickel, 2.2 percent titanium, metals such as aluminum and/or zirconium. and 0.3 percent aluminum, showed no appreciable These elements may be associated with each hardening when air-cooled from 1000 degrees other in any desired amounts to givecomposicentigrade. A similar alloy including 34 percent 50 i tions having certain specified characteristics. nickel, 2.5 percent titanium, and 1.9 percent alul The preferred range of titanium .is substanminum increased in hardness about 110 Brinell tially from 1 to4 percent in alloys in which units on air-cooling. Both alloys hardened to metals of the iron and chromium groups preabout 320 Brinell units when furnacecooled. It i dominate, but titanium contents even below 1 will also be appreciated that by the use of hardpercent may be desirable in alloys in which copening agents in multiple as hereindescribed, it is per predominates. This range is determined ap-' possible to secure marked economies in manufacproximately by the first appearance of hardenture due to the ability to use cheaper addition ing and the substantial disappearance of hot malmaterials -without in any way sacrificing the leability. When it is desired to retain good hot good results desired in the finished product. 60

- and cold working properties in order to permit The diversity of base compositions amenable shaping by forging, hot rolling, cold rolling, to hardening by titanium and aluminum has been drawing, or plastic deformation generally, full described. No common alloying elements in advantage cannot be taken of the maximum tiamounts less than 2 percent have been found to 0.3 tanium contents In such cases it is preferable interfere with this hardening characteristic with to employ titanium contents ranging from 2.21 to the exception'of aluminum and carbon. The ef- 3.2 percent for alloys having a lowcarbon confect of aluminum when combined with titanium tent and in which the base is nickel-iron, nickelhas just been described. Since carbon forms an copper-iron, and nickel-chromium-iron. It will, inert titanium carbide, itspresence with titanium of course, be understood that in case of castings is highly detrimental; This is due to the fact where workability is not a factor to be considthat although the total titanium content may be ered, a much greater range of titanium is pergreat enough to indicate vigorous hardening, the mitted with a correspondingly greater degree of alloy is, in fact, devoid of hardening response, hardness. In such cases as much-as 10 percen It is highly desirable, therefore, to keep the ca titanium may be used to advantage. bon content as low as is metallurgically feasible.

Alloys of this type have been produced with as little as .01 percent carbon, yet melts containing as much as 0.40 percent carbon have been produced which displayed good hardening properties, although an inefliciently high titanium content in the alloy was necessary.

It is considered to be within the scope of this invention to provide, in addition to the major elements of composition; such other elements as are commonly used in metallurgy to aid in refining, purifying, degasifying, and otherwise treating the alloy to insure its production in sound, tough, malleable form. These auxiliary elements are:

Percent Manganese up to Silicon up to 5 Aluminum up to 1 Vanadium up to 1 Zirconium up to 1 Titanium up to A; CaICiUIXL. up to Magnesium up to /2 Boron up to /2 the softest working condition, the heat treatment required in all cases is a not too slow cooling from above a minimum temperature. Most efiicient results are obtained when this minimum temperature is exceeded, but the temperature margin by which it is exceeded is not of very great importance, the upper limit usually being that at which an undesirable coarsening in grain size occurs. The minimum softening temperature varies directly with increase in content of the hardening element or elements, and also varies to some extent with the composition of the base alloy. For contents of titanium and/or aluminum which yield malleable alloys, this minimum temperature is generally from'750 degrees centigrade to 850 degrees centigrade, and can easily be established for a particular alloy. As a general rule the entire group of alloys herein described respond well to a range of softening temperatures varying from 900 degrees C. to 1050 degrees C. The rate of cooling required to avoid hardening is not great, and air cooling will usually prove fast enough, although cooling in water or in oil is permissible.

Where it is desired to heattreat the alloys in order to harden them, the treatment is much more variable. Variations in composition of the base metal, and of the hardening elements affect both the temperature at which the desired hardening is most effectively produced, and also the rate at which it occurs. In all cases hardening occurs over a considerable range of temperatures, and the lower the temperature at which this can be carried out, the greater will be the hardness ultimately developed. Since the rate of hardening diminishes as the temperature is decreased,

an optimum hardening temperature may be appropriately designated.

With a hardening treatment which includes holding the alloy at a fixed temperature for several hours, the preferred hardening temperature is substantially 700 degrees C; for alloys in which titanium is the hardening element, and about 600 degrees C. when aluminum or zirconium is the hardening element. It is to be noted that when chromium does not exceed about 5 percent, good hardening may be produced by furnace cooling from the softening range. When the chromium content exceeds this value, the hardening reaction proceeds sluggishly, and considerably more time is required in order to develop full hardness. High chromium alloys containing up to 30% chromium may show very little hardening on furnace cooling. Alloys in which copper is a predominating element may exhibit optimum response to heat treatment at temperatures even below that designated above, for example 400 to 500 C. in the case of copper-nickel titanium alloys.

When it is desired to develop the maximum hardness of a given alloy, it has been found advantageous to carry out the hardening operations in several steps at progressively lower temperatures and preferably with the duration of heating increasing at the lower temperatures. The temperature range in which this incremental hardening may be carried out is from the minimum softening temperature above described, down to about 400 C. or about 500 C. As a particular example, an alloy of a composition including Percent Ni 22.0 Cr 6.7 Ti 2.6 Fe Bal.

having an initial Brinell hardness of 148 hardened to 290 Brinell after twenty-four hours of treatment at I00 degrees C. When an incremental hardening heat treatment was given to this alloy, a hardness of 340 Brinell units was secured, the particular treatment included heating at 750 degrees C. for two hours, followed by heat treatment at 680 degrees C. for five hours, and at 600 degrees C. for twenty-three hours.

On the other hand, for the purpose of improving toughness and ductility of the hardened alloy, the termination of the hardening operation may include the step of reheating to a temperature higher than the last preceding step, but still within the range of temperatures in which the particular a loy is hardenable.

A further example may be given in which the hardening characteristics as described hereinabove are combined with martensitic hardening of the type commonly observed in air-hardening steels. This combination occurs in marginal austenitic nickel-content ferrous alloys of the nickel, nickel-chromium, nickel-copper, nickelmanganese and related series in which the iron content is up to about 10 percent lower than'that at whichmartensite ceases to be a constituent under ordinary conditions of cooling.

Alloys of the aforesaid type when heat treated develop a strengthening precipitate, accompanied by a change in composition of the residual matrix sufficient to shift the latter within the range of compositions which have a true allotropic transformation, and hence, at suitable cooling velocities, can be transformed at least partially into martenslte. The following are two examples of alloys in which the effect is characterized by intense hardening.

5 Brinsll hardness number Ni Cr '1! 0 Al 10000. water Tern red quenched 15 These alloys were both completely austenitic and nonmagnetic when in the soft condition, but became magnetic and partially martensitic after heating between 600 and 700 C. followed by cooling in the air.

As exemplifying the physical properties produced in malleable alloys of the type under consideration, the following table is included:

fact that the present hardening elements may add hardening properties to particular base alloys without detriment to their other distinctive properties, thus affording a combination of strength with other special-qualities not prevl- 5 ously possible. For example, the addition of titanium to austeniticnickel-chromium steels irn parts hardness and high elastic properties without interfering with the.valuable corrosion and heat resisting qualities of the latter. In particu- 10 I lar cases 1in which a property is ,closelyassociv ated with'a specific nickel content, e. g., low ex-,

' Pro. Ult. Elong. Red 5 No Ni Cu 01' TI Fe Temper limfi; strength percent r 5 psi psi 2" percent I 25 30.1 s 0 2 5 Hal. Soft 24,800 01, 00 01.5 05.1

Hardl 110,000 102,000 10.0 33.0 20 21.7 0.3 2.4 Bal. Soft 34,000 87,800 35.0 59.8 30 Hard 77,400 158,000 22.0 47.8

15.5 12.4 2.7 Bal. Soft 88,000 48 68 102 Hardin.) 00,000 100,000 30 02 74 Hard(b) 7 00,000 100,000 22 18 00 35 (rd-700 O. temp.

(b)-Incremental temper.

In addition to exhibiting these high physical properties at room temperatures, the high 40 strength and elastic properties shownnray be retained at high temperatures, provided that the base alloy is of a suitable type; iron-nickelchromium is appropriate, and the titanium alloy with this base shows excellent strength proper- 90,000 psi PL. 125,000 psi ULT. 25% Elong. in 2'.

12% red area involving considerable heat and load such as obmm in steam and, internal combustion turbines, as well as in many chemical processes, a particu- 60 lar example being that of tube stills and like apparatus which may be used in oil-cracking and oil refining.

Many alloys, in particular steels, exist which have hardness and tensile properties equal to or 65 even excelling the alloys of the present type.

' The advantage of the latter lies in the unique titanium, aluminum and/or zirconium. It is .to

be noted further that the hardenable alloys comprehended within the spirit and scope of the present invention, are adapted for a wide variety of uses, and more particularly for use in structures which can economically be made by plastic deformation such as drawing, pressing, etc., such formed articles being adapted to being suitably hardened by a heat treatment as set forth.

The present application is acontinuation-inpart of our c'o-pending application, Serial No. 356,870, filed April 15, 1929, entitled-"Titanium alloy. 50

What is claimed is:

The process of producing a hard alloy composed of a base substantially of iron but which may containsmall amounts of the usual iron alloying ele- ,.nients and from 3 to 6% of titanium, which Such alloys are particularly suited for purpose sa process consists in the steps of heating thealloy to an; elevated temperature below its melting point but sufllciently high to cause the age hardening element to go into solution with the iron, quenching the alloy and reheating to a temperature below that of the: initial heating but sufficiently high and for a period of time sumcient to obtain a substantialincrease in the hardness of the alloy.

J NORMAN B. FILLING.

PAUL DYER. MERTCA. 

